1. Field of the Invention
The present invention relates to a highly reliable MOS transistor devices wherein a semiconductor film and metallic film comprising fine crystals are used as a gate electrode.
2. Description of the Related Art
Gate oxide films in MOSFETs (Metal Oxide Semiconductor Field Emission Transistor, hereinafter, abbreviated as “MOS transistor”) have successively been reduced in thickness on the basis of the scaling rule proposed by R. Dennard, but at thicknesses of 3 nm or less, there have been concerns regarding the increase in leakage current due to tunnel effects at low voltages. However, it has been confirmed that even if a tunnel current is flowing, a MOS transistor devices can be made to operate normally by shortening the gate length, and MOS transistor devices using gate oxide films of 1.5 nm or less have been achieved. Investigation has also been conducted into oxide equivalents of 1.0 nm or less, wherein materials such as silicon nitride, tantalum oxide, or the like, having higher dielectric constant than silicon oxide film are used for the gate insulating film. In an ultra-thin oxide film of this kind, there is significant increase in capacitance due to depletion in the gate electrode, and suppression of such depletion is a problem.
With this reduction in the thickness of gate insulation films, it has been essential to adopt new materials generating less depletion, instead of conventional impurity-doped polysilicon films, as the gate electrode material. In order to reducing this depletion, methods using metallic gates have been investigated, but processing problems relating to interface control, and the like, have arisen, and such methods have not yet been achieved. On the other hand, in methods for reducing depletion by using the same process currently adopted for polysilicon, attention has been focussed on polysilicon germanium films.
Gate depletion is particularly marked in PMOS devices using boron having a low activation rate, but with polysilicon germanium, since the boron active concentration is higher than in polysilicon, it is possible to suppress depletion.
When a polysilicon germanium film is formed on an oxide film, the crystal grains enlarges, leading to deterioration in the breakdown voltage of the gate insulating film. Accordingly, Wen-Chin Lee et. al., for example, in IEEE Electron Device Letters, Vol. 19, No. 7, p.247-249, have used a method wherein a silicon film is inserted in order to achieve finer crystal grains. Moreover, at the International Electron Device Meeting 2000, Ueshima, et. al., presented a method for improving reliability by providing a 3 nm amorphous silicon film as a barrier layer at the interface between polysilicon germanium and oxide film, thereby improving the gate oxide film interface.